It is known to purge a bed of adsorbent of gaseous impurities using product gas before recovering the preferentially adsorbed gas from such material, in order to achieve a better degree of purity of the desorbed product gas. During adsorption, a certain amount of the less readily adsorbed components of the feed gas mixture will remain in the interstices of the bed and indeed some of such components could also be loosely held by the adsorbent material. The object of the purge with product quality gas is to remove such components from the bed so that they are not withdrawn with the desorbed product gas as contaminants thereof. Another result of the purge operation is to achieve the saturation value for the adsorbent material with the preferentially adsorbed gas at the particular temperature and pressure of the bed after purging.